BACKGROUND/PURPOSE: The purpose of this study was to assess the tracheoscopic placement of a detachable balloon to obstruct the trachea in the fetal lamb model and to study the pulmonary effects 2 weeks later. METHODS: Thirteen pregnant ewes carrying 27 fetuses were used in this trial. Thirteen fetuses underwent endotracheal plugging at a mean gestational age of 95 (90 to 100) days (term, 145). Fourteen nonoperated fetuses served as controls. The intended duration of obstruction was 15 days. Study parameters consisted of operating time, fetal survival, performance of the balloon, and pulmonary changes. RESULTS: Hysterotomy time was 41 +/- 25 minutes and tracheoscopy time was 7.2 +/- 3.4 minutes. There were no intraoperative deaths in this series. Postoperative fatalities occurred in 2 of 13 plugged fetuses and 1 of 14 controls. At completion of the experiment, the balloon was recovered intact in the trachea of 12 of 13 (92.3%) fetuses. The lung weight/body weight ratio was 76 +/- 16 mg/kg for plugged animals, while in controls it was 34 +/- 8 mg/kg (P = .0001). The lung volume/body weight ratio was 101 +/- 17 mL/kg in plugged fetuses, compared with 47 +/- 4 mL/kg for controls (P = .0002). Mean-terminal-bronchial-density was 0.937 +/- 0.303 for plugged animals, compared with 1.911 +/- 0.441 for controls (P = .0002). Mean linear intercept was 60.9 +/- 5.2 microm in treated fetuses, and 46.3 +/- 5.8 microm for controls (P = .0004). CONCLUSIONS: Fetal endotracheal occlusion, using a tracheoscopically placed detachable balloon, is reliable, and results in pulmonary hyperplasia after 2 weeks of obstruction. This technique may be beneficial in treating the pulmonary hypoplasia seen in congenital diaphragmatic hernia. Further studies using this balloon device are now required to corroborate these findings.